Compositions containing polydiorganosiloxanes, which can be cured to form elastomeric foams are well known in the art. The foams may be prepared by adding gas-generating additives to the curable compositions to produce uniform foaming of the compositions during the curing reaction. For example, in the curing reaction catalyzed by platinum or platinum compounds and based on polydiorganosiloxanes containing hydroxyl groups and polyorganohydrogensiloxanes, hydrogen is evolved, which initiates the foaming. In addition, curable compositions of this type can contain, for example, up to about 85 percent by weight of polydiorganosiloxanes having vinyl groups, which usually result in improved strength properties in the finished foams, as well as other additives which are generally present in platinum catalyzed compositions, such as fillers and curing inhibitors, for example, acetylene compounds, which are intended to prevent premature curing and to prolong the pot life of the compositions.
In U.S. Pat. No. 3,925,705 to Smith the polydiorganosiloxanes containing hydroxyl groups act as a "blowing agent" and are responsible for the formation of hydrogen as the "blowing gas". However, since these compounds are themselves polymers and are directly involved in the curing reaction as a result of crosslinking by means of the condensation reaction, it is difficult to control the foam formation under these conditions.
Improved foam control can be achieved if the polydiorganosiloxanes which contain hydroxyl groups and act as blowing agents in the compositions catalyzed by platinum are replaced wholly or in part by organic monohydric alcohols, and if the curing reaction takes place between the polydiorganosiloxanes containing vinyl groups and polyorganohydrogensiloxanes by means of the addition reaction such as described in U.S. Pat. Nos. 4,026,843 to Kittle and 4,026,844 to Kittle et al. In addition, it is also known from U.S. Pat. No. 4,189,545 to Modic that water can be used as the compound which contains hydroxyl groups in compositions of this type.
However, when mixed with monohydric alcohols, especially lower alcohols, curable compositions containing polyorganosiloxanes are flammable and additional safety measures are required when these compositions are processed. Polyorganosiloxane mixtures containing water, on the other hand, have only a limited stability on storage because of their poor miscibility. These disadvantages are not eliminated when the crosslinking is reduced in these curable compositions by selecting particular polyorganohydrogensiloxanes (cf. DE-A No. 3,443,677, which corresponds to EP-A No. 183,148).
Therefore, it is an object of the present invention to provide compositions which can be cured to form elastomeric foams. Another object of the present invention is to provide compositions having improved foam control. Another object of the present invention is to provide compositions which can be cured to form elastomeric foams of uniform pore structure. Still another object of the present invention is to provide highly flexible elastomeric foams having low combustibility. A further object of the present invention is to provide a method for controlling the foam density.